Why evolution often favors small animals and other organisms

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Small seems really nice from an evolutionary perspective. The largest dinosaurs, pterosaurs and mammals may look impressive, but these giants are vastly outnumbered by microscopic bacteria and single-celled algae and fungi. Small organisms are also ancient and incredibly resilient.

The first evidence of single-celled organisms dates from about 3.8 billion years ago, shortly after the newly formed Earth had cooled enough for organic life to emerge. Multicellular animals evolved less than a billion years ago, while larger and more complex animals appeared just over half a billion years ago. For most of Earth’s history, the planet was dominated by organisms no larger than the diameter of a single human hair.

Large animals generally take longer to grow and mature, so they reproduce more slowly. While mice have a short generation time (how long it takes for a newborn to grow and give birth) of about 12 weeks, elephants take closer to 25 years.

Large species tend to evolve more slowly and may be less able to cope with longer-term changes in the physical and biological environment. Larger organisms also tend to fare worse in mass extinction events. Nothing much bigger than a domestic cat survived the asteroid impact that wiped out the dinosaurs 66 million years ago.

Being very large requires much more specialization and slower reproduction, both of which reduce the chances of surviving environmental upheavals. For example, larger vertebrates require disproportionately thicker bones and larger muscles. A shrew the size of an elephant would quickly break its legs if it tried to walk.

So it is not surprising that many groups of animals appear to be of relatively small size, and the earliest branching representatives tend to be quite small. Sister groups to winged insects include the tiny springtails (usually less than 6 mm), while the microscopic tardigrades or ‘water bears’ are the sister group to arthropods (including spiders and crustaceans) and velvet worms.

The earliest mammals and some of the earliest dinosaurs (such as Eoraptor with a length of less than two meters) were also relatively small compared to their later, often gigantic cousins.

Why bother getting bigger at all?

There are many advantages to being bigger. Larger size can make it easier to avoid predators (elephants and whales have few enemies other than humans), hunt prey, defeat rivals, and endure temporary hardship.

Larger organisms also tend to be better at retaining heat (due to their relatively smaller surface area) and have greater potential for intelligence.

But scientists believe there is an upper limit to cell size. The mechanisms of cell division break down at very small and very large sizes.

All living things also face a universal physical limitation as noted by Galileo Galilei. Larger cells generally have a smaller surface area per unit volume. This means that the natural movement (diffusion) of molecules of gases, nutrients and waste in and out of the cell is not enough to keep everything running without a transport system. These molecules also have to move further into larger cells.

So building a larger organism involves two things. First, grouping many cells together so they can work together. Second, specializing different cells for different tasks, including structural support, digesting food, and moving things like oxygen and CO₂.

The alternative is to become flat or threadlike (like horsehair worms) or thin and flat (like flatworms). These animals do not require an internal transport system because none of their cells (or their contents) are far removed from the surrounding air or water.

The paleontologist Edward Cope (1840-1897) postulated that individuals within all lineages increase in size over evolutionary time. While this is true in a statistical sense, there are many exceptions, and mass extinction events often move things to the smaller end of the spectrum.

Draw the size distribution for almost any major group of animals and you’ll discover a strikingly positive skew: most species are much closer to the smallest size than the largest size within their parent group, and there are relatively few large species. For example, there are more species of insects (about 5 million) than all other groups of animals combined, making them perhaps the most successful group of animals on Earth.

Most insects are beetles, with an average body length of about 6 mm. Giants such as the Hercules (17 cm long) and elephant beetles (13 cm long) are extremely rare.

Their small size allows animals to live in a greater diversity of niches and distribute resources more finely, allowing more species and individuals to be accommodated in the same habitat space. Insects are masters at this strategy.

The meek will inherit the earth – and beyond

Despite the tendency of organisms to evolve to larger sizes, the simplest and smallest organisms still have many incredible abilities that larger organisms lack.

Many of these tiny “extremophiles” can survive environments that wipe out most other life forms.

Some archaea (single-celled organisms without a nucleus) can withstand temperatures of more than 200°C around deep-sea vents, while other species can thrive in water with high salt, acid, and alkaline concentrations. Likewise, the small tardigrades can withstand temperatures between 150°C and -200°C, the vacuum of space, which dries out for decades, and radiation doses a thousand times higher than those needed to kill a human .

Read more: The secret world of moss, ancient ancestor of all plants and vital to the health of the planet

There are even small nematode worms that can live under three kilometers of solid rock.

Some scientists think microbes could survive interplanetary travel in meteorites. Scientists also think that all life found elsewhere in the solar system may have a common origin with life on Earth – which starts small.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Matthew Wills has received funding from BBSRC, NERC, The Leverhulme Trust and The John Templeton Foundation.

Tim Rock does not work for, consult with, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond his academic appointment.

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